Stainless Steel Pipe Fittings

Causes and preventive measures for defects of stainless steel welded pipe

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Welding defects in stainless steel welded pipes can lead to stress concentration, reduced load carrying capacity, shortened service life, and even brittle fracture. General technical regulations stipulate that cracks, incomplete penetration, unfusion and surface slag are not allowed; defects such as undercut, internal slag and pores cannot exceed certain allowable values, and must be thoroughly removed and welded for excessive defects. Make up. The causes, hazards and preventive measures of welding defects of common stainless steel welded pipes are briefly described below.
First, the weld size does not meet the requirements
The weld seam size does not meet the requirements mainly refers to the weld height and residual height difference, the weld width and width difference, the wrong side amount, the post-weld deformation amount, etc., which are not in conformity with the standard. The weld is uneven, uneven, and deformed. Larger and so on. The weld width is inconsistent, in addition to causing the weld bead to be unsightly, it also affects the bond strength between the weld and the base metal; if the weld height is too large, resulting in stress concentration, and the weld is lower than the base metal, there is not enough Joint strength; if the wrong side and deformation are too large, the force transmission will be distorted and stress will be concentrated, resulting in a decrease in strength.
Causes: improperly angled or unblind edge of stainless steel welded pipe and uneven assembly clearance; unreasonable selection of welding process parameters; low skill level of welders.
Precautionary measures: select the appropriate groove angle and assembly clearance; improve the assembly quality; select the appropriate welding process parameters; improve the welder’s operating technology level.
Second, undercut
Due to incorrect selection of welding process parameters or incorrect operation process, the grooves or depressions formed by burning along the base material of the weld toe are called undercuts. The undercut not only weakens the strength of the welded joint of the welded pipe, but also causes cracks due to stress concentration.
The cause is: the current is too large, the arc is too long, the angle of the electrode is not correct, and the method of transporting the strip is improper.
Preventive measures: When welding arc welding, it is necessary to select the appropriate welding current and welding speed, the arc can not be pulled too long, the angle of the electrode should be appropriate, and the method of transportation should be correct.
Third, not welded
Incomplete penetration refers to the phenomenon that the root of the welded joint is not completely penetrated during the welding of the stainless steel welded pipe. Incomplete penetration can cause stress concentration and can easily cause cracks. Important weld joints are not allowed to pass through.
The reason is: the groove angle or gap is too small, the blunt edge is too large, the assembly is poor; the welding process parameters are improperly selected, the welding current is too small, the welding speed is too fast; the welder has poor operation technique.
Precautionary measures: Correctly select and process the groove size, properly assemble, ensure the clearance, select the appropriate welding current and welding speed, and improve the technical level of the welder.
Fourth, not fused
Unfused refers to the portion of the weld bead and the base metal or between the weld bead and the weld bead that is not completely melted during fusion welding. Unfused directly reduces the mechanical properties of the joint, and severe unfused causes the welded structure to be unsupportable at all.
Causes: Mainly when welding stainless steel welded pipe, the speed is fast and the welding current is small, the welding heat input is too low; the welding rod is eccentric, the angle between the welding rod and the welding piece is improper, the arc is deflected; the sidewall of the groove has rust and dirt, and the layer The slag is not completely cleaned.
Preventive measures: Correctly select the welding process parameters, carefully operate, strengthen the interlayer cleaning, and improve the technical level of the welder.
Five, welding tumor
A weld bead is a metal tumor formed by a molten metal flowing into an unmelted base metal outside the weld during welding. The weldment not only affects the formation of the weld of the stainless steel welded pipe, but also often has slag inclusions and no penetration in the weld.
The reason is: the blunt edge is too small and the root gap is too large; the welding current is large and the welding speed is fast; the welder’s operation skill level is low.
Preventive measures: According to different welding positions, it is necessary to select appropriate welding process parameters, strictly control the size of the molten holes, and improve the technical level of the welder.
Six, crater
The depressed part of the industrial grade stainless steel welded pipe weld end is called the crater. The crater not only severely weakens the strength of the weld at that location, but also causes crater cracks due to the concentration of impurities.
Cause: The main reason is that the arc extinguishing time is too short; the current is too large when the thin plate is welded.
Preventive measures: When welding arc welding, the welding rod should be slightly stopped at the molten pool or used as an annular moving strip. After the molten metal is filled, it will be led to one side to extinguish the arc. When tungsten argon arc welding is required, there should be enough The residence time, after filling the weld, decays the arc.
Seven, stomata
When welding industrial grade stainless steel welded pipes, the holes formed by the gas in the molten pool that cannot escape during solidification and remain are called pores. Pores are a common weld defect that is divided into internal welds and external air holes. The pores are round, elliptical, worm-shaped, needle-shaped, and dense. The presence of pores not only affects the tightness of the weld, but also reduces the effective area of the weld and reduces the mechanical properties of the weld.
Causes: There are oil, rust, moisture and other contaminants on the surface and groove of industrial grade stainless steel welded pipe; when the welding rod is arc welding, the coating strip is damp, not dried before use; the arc is too long or partial blow, the molten pool protection effect is not Well, the air invades the molten pool; the welding current is too large, the welding rod is red, the coating peels off in advance, and the protection is lost; the operation method is improper, such as the arcing action is too fast, the shrinkage hole is easy to occur, the joint arcing action is incorrect, and it is easy to produce Dense pores, etc.
Preventive measures: Remove the oil, rust and moisture in the range of 20~30mm on both sides of the groove before welding; strictly bake according to the temperature and time specified in the specification of the welding rod; correctly select the welding process parameters and operate correctly; try to use short arc Welding, wind-proof facilities in the field construction; failure to use welding rods, such as corrosion of the core, cracking of the coating, peeling, excessive eccentricity, etc.
Eight, inclusions and slag inclusions
Inclusions are non-metallic inclusions and oxides that are produced by metallurgical reactions in the weld metal. The slag inclusions are slag remaining in the weld. Stainless steel welded pipe slag can be divided into two types: point slag and strip slag. The slag inclusion weakens the effective section of the weld and reduces the mechanical properties of the weld. The slag inclusion also causes stress concentration, which easily causes the welded structure to be damaged during loading. Cause: The slag cleaning in the welding process is not clean; the welding current is too small; the welding speed is too fast; the welding process is improper; the welding material and the parent metal chemical composition are not properly matched; the groove design and processing are not suitable.
Preventive measures: Select the welding rod with good slag removal performance; carefully remove the interlayer slag; reasonably select the welding process parameters; adjust the welding rod angle and the moving strip method.
Nine, burn through
During the welding process, the molten metal flows out from the back of the groove, and the defect in which the stainless steel welded pipe forms a perforation is called burn through. Burn through is one of the common defects of electrode arc welding.
Cause: The welding current is large, the welding speed is slow, and the welded pipe is overheated; the groove gap is large, the blunt edge is too thin; the welder has poor operation skill.
Preventive measures: select the appropriate welding process parameters and the appropriate groove size; improve the welder’s operating skills.
Ten, crack
Industrial grade stainless steel welded pipe cracks can be divided into cold cracks, hot cracks and reheat cracks according to the temperature and time of their generation; according to the different parts they produce, they can be divided into longitudinal cracks, transverse cracks, root cracks, crater cracks, Fuse line cracks and heat affected zone cracks. Cracks are one of the most dangerous defects in a welded structure, which not only causes the product to be scrapped, but may even cause serious accidents.
(a) hot crack
During the welding process, the weld cracks generated by the weld and the heat-affected zone metal cooling to the high temperature zone near the solidus line are called hot cracks. It is a dangerous soldering defect that is not allowed to exist. According to the mechanism, temperature interval and shape of the hot crack of the welded pipe, the hot crack can be divided into crystal crack, high temperature liquefaction crack and high temperature low plastic crack.
Causes: mainly the low melting point eutectic and impurities in the molten pool metal during the crystallization process, forming severe intragranular and intergranular segregation, and under the action of welding stress. It is pulled apart along the grain boundary to form a thermal crack. Hot cracks generally occur in austenitic stainless steels, nickel alloys, and aluminum alloys. When low carbon steel is welded, it is generally not easy to generate hot cracks, but as the carbon content of the steel increases, the hot cracking tendency also increases. Preventive measures: Strictly control the content of harmful impurities such as sulfur and phosphorus in stainless steel welded pipes and welding materials, reduce the sensitivity of hot cracks; adjust the chemical composition of weld metal, improve weld microstructure, refine grains, improve plasticity, and reduce Or disperse the degree of segregation; use alkaline welding materials to reduce the content of impurities in the weld and improve the degree of segregation; select appropriate welding process parameters, appropriately increase the weld forming coefficient, and adopt multi-layer multi-pass welding method; Use the same lead plate as the base material, or gradually extinguish the arc and fill the crater to avoid hot cracks in the crater.
(2) Cold crack
Cracks that are generated when the welded joint is cooled to a lower temperature (below M. for steel) are called cold cracks. Cold cracks can occur immediately after welding, and may occur after a period of time (hours, days, or even longer). This crack is also called delayed crack. It is a common form of cold crack and has more Great danger.
Causes: The hardened structure formed by martensite transformation, the welding residual stress formed by the large degree of restraint, and the hydrogen remaining in the weld are the three major factors that cause cold cracking.
Preventive measures: Use low-hydrogen type welding materials, strictly follow the instructions in the instructions before use; remove the oil and moisture on the weldments before welding, reduce the hydrogen content in the weld; select reasonable welding process parameters and heat input to reduce The tendency of hardening of the weld; the hydrogen dehydration treatment is carried out immediately after the welding to allow hydrogen to escape from the welded joint; for the stainless steel welded pipe with high tendency to harden, preheating before welding and heat treatment after welding to improve the structure and performance of the joint ; Various process measures to reduce welding stress.
(3) Reheat crack
After welding, the crack generated by the stainless steel welded pipe reheating in a certain temperature range (stress-relieving heat treatment or other heating process) is called reheat cracking.
Cause: Reheat cracks generally occur in low-alloy high-strength steels, pearlitic heat-resistant steels and stainless steels containing alloying elements such as vanadium, chromium, molybdenum and boron. After undergoing a thermal cycle of welding, they are heated to a sensitive area (550). ~ 650 ° C) produced. Most of the cracks originate from the coarse-grained areas of the heat affected zone of the weld. Most of the reheat cracks occur in stainless steel welded pipes and stress concentrations, and reheat cracks sometimes occur in multilayer welding.
Preventive measures: Under the premise of meeting the design requirements, select low-strength welding materials, so that the weld strength is lower than the base metal, the stress is loose in the weld, avoid cracks in the heat-affected zone; minimize welding residual stress and stress concentration; Control the welding heat input of the welded pipe, reasonably select the preheating and heat treatment temperature, and avoid the sensitive area as much as possible.
Source: China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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Steel Pipe

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